is there a way to change from child to adult account on iCloud

The difference between child and adult accounts is the birth date set for the account.

If your account is set to under 18 years of age the account is considered a child.

You need to edit the birth date of the account to change that: Update the birth date associated with your Apple ID - Apple Support

Do note that in Family Sharing and specifically for purchase sharing whoever is set as the organizer will be required to pay for every purchase made by every member of the family group, and will be required to have a credit card on file on their account.

If you are not actually an adult, and don't have a credit card, this is not something you want to be doing.

See these articles Family Sharing and Apple ID for your child - Apple Support

Help your child set up an iPhone, iPad, or iPod touch - Apple Support

Set up Family Sharing

With Family Sharing, you can share each other’s App Store, iTunes, and Apple Books purchases, an Apple Music Family Subscription, iCloud storage, and more, without sharing an account. And when you turn on purchase sharing, any purchases initiated by your family are billed to your account.

• If your child already has an Apple ID, you can set up Family Sharing now.
• If your child needs an Apple ID, you can create one for them with Family Sharing.